Average First Freeze & Frost Dates

Dew and Frost are the results of what is known as radiational cooling. At night, outside objects will release heat into the atmosphere, thus cooling the object. At the same time, the atmosphere will radiate heat back into the object. But on a clear, calm night the amount of heat loss tends to be higher and the object can become cooler than the surrounding air.

With sufficient cooling, the air adjacent to the object can cool to its dew point temperature (a measure of the air’s water vapor content) and become saturated. If the air temperature is above freezing, the result will be the formation of dew. However, if the temperature is near or below freezing, frost will develop.

Cold air is more dense than warm air and tends to sink. On a calm night, air will sink into lower elevations, creating a temperature "inversion" that keeps the colder air near the surface. Under such conditions, valleys can be much colder than the surrounding higher terrain.

On calm, clear nights, the temperature near the ground can be three or four degrees cooler than just a few feet above. Most temperature measurements are made about 6 feet above the ground, so even if the thermometer reads about 35 or 36 degrees, the temperature near the surface can be very close to the freezing point.